Impact deposition of particulate materials

ABSTRACT

Method of and apparatus for the high-energy rate deposition of particulate materials upon a receiving surface whereby the particles are preheated, preferably concurrently with their formation from a coherent body by subjecting the body to a plasma or electrically fusing the body, and projected against the substrate by intermittent spark discharge, a discharge electrode for this purpose being located behind the particle cloud in the direction of propagation of the particles. Alternatively, encapsulated doses of the particles or masses thereof may successively be disposed in the path of the discharge electrode upon a rotatable turret or disc.

United States Patent Kiyoshi Inoue No. l82-3-Chome Tamagawayoga-Machi,Selagaya-ku, Tokyo. Japan [72] Inventor 21 AppLNo. 696,757

[22] Filed Jan. 10, 1968 [4S] Patented Jan. 5, 1971 32 Priority Jan. 17,1967, Aug. 4, 1967, Aug. 16,1967 [33] Japan [3 1 42/3,537, 42/50042 and42/52563 Continuation-impart of application Ser. No. 574,056, Aug. 22,1966, now Patent No. 3,461,268, and a continuation-in-part of 629,633,Apr. 10, 1967, now Patent No.

[54] IMPACT DEPOSITION 0F PARTICULATE MATERIALS 6 Claims, 10 DrawingFigs.

52 11.5. C1 239/81, 117/105, 118/308, 72/56 [51] Int. Cl 844d 1/52 [50]Field ofSearch 239/15,81, 79; 117/17, 105; 1 18/308; 72/56 [56]References Cited UNITED STATES PATENTS 2,714,563 8/1955 Poorman etal..... 117/105 2,869,924 1/1959 McGill 239/81 3,212,914 10/1965 Lyle etal. 118/308X Primary Examiner- M. Henson Wood, Jr. AssistantExaminer-Michael Y. Mar Attorney- Karl F. Ross ABSTRACT: Method of andapparatus for the high-energy rate deposition of particulate materialsupon a receiving surface whereby the particles are preheated, preferablyconcurrently with their formation from a coherent body by subjecting thebody to a plasma or electrically fusing the body, and projected againstthe substrate by intermittent spark discharge, a discharge electrode forthis purpose being located behind the particle cloud in the direction ofpropagation of the particles. Alternatively, encapsulated doses of theparticles or masses thereof may successively be disposed in the path ofthe discharge electrode upon a rotatable turret or disc.

PATENTEI] JAN Si n 3,552,653

SHEET 1 0f 3 6/0 r-& 6/2

5I08 I Kwosm INOUE INVENTOR.

BY CK,- g Tm Lttomey PATENTEU JAN SIQYI 3552' 653 SHEET 2 BF 3 AirFlG.3A

m9 1/50 7 m9 I50 00 X /OO FIG.3C F563 200 m9 I00 mg /50 50 c KIYOSHI IINOUE 5 INVENTOR.

BY M g Alton; y

PATENTEU JAN 5197:

sum 3 or 3 FIG KIYCSHI INOUE INVENTOR.

BY Qu'l Atto y This application is a continuation-in-part of mycopending applications Ser. No. 574,056, filed 22'.Aug. 1966, and Ser.

No. 629,633, filed Apr. 1967 (now Pat-.No. 3,461,268.

ln my application Ser. No. 574,056, which is a continuationin-part ofapplication Ser. No. 31 1,061 (now U.S. Pat. No. 3,267,710) andapplication Ser. bio/508,487, filed 18 Nov. 1965 as acontinuation-in-part ofapplication Ser. No. 41,080, (now U.S. Pat.3,232,085), I have pointed out that metallic substrates and othersurfaces may be-coated with surface layers of a pulverulent material in'a convenient, economical and satisfactory manner when a sourceo fdetonation-type impulsive waves is juxtaposed with a surface of the bodyto be coated and between this body and the source, a mass of pulverulentmaterial is placed (preferablyfin proximity to the substrate, therebyensuring the improved bond between'the coating material and thesubstrate. The heating means there detonation source). Thepulverulentrnaterial can have a hardness greater than'that of thesub'strate and may-even be nonbondable thereto by conventional methods.The detonationtype wave described in that application was generated'byan impulsive, intermittent spark discharge and apparently pro-.

jected the particles onto the substrate with a velocity (and kineticenergy) sufficient to overcome the rebound tendency at the surface andto cause the particles to lodge thereon with a firm bond to thesubstrate. The technique is particularly advantageous when applied tothe bondingjof particles of a hard facing material (e.g. tungstencarbide) or hard-alloy steels to metallic, synthetic-resin or likesubstrates.

In that application, a particularlyadvantageous system was describedwherein the particulate: material was a layer of powder disposed upon orin a frangible foil, film or sleeve juxtaposed with the surface to becoated and forming a rupturable diaphragm retaining the particle layerand separating a discharge chamber" from the workpiece chamber orpropulsion path. The latter chamber is vented to the'atmosphere via asound-damping muffler to prevent the development of substantial outwardpressure within the [workpiece chamber which might resist the highvelocity movement of the particles as well as to destroy the violentsound wavewhich such discharges have a tendency to develop. The use of afrangible diaphragm to retain the particles in this manner facilitatesthe uniform deposition of the particles upon the surface, especiallywhen the diaphragm is generally parallel to the surface of the substrateto be coated or conforms to the latter. Moreover, the diaphragmconstituted 'the counterelectrode for the spark-discharge system formingthe detonation source. The other discharge electrode was'a needle spacedfrom and perpendicular to the frangible diaphragm. The apparatuspreferably made use of a discharge chamber in the form ofa gun or shocktube whose barrel was trained upon the workpiece and received, at anintermediate location therealong, a

mass of particles which were propelled against the surface of thesubstrate upon triggering of a spark-type discharge at the closed end ofthe barrel. in the horizontal position of the barrel, the particles wereintroduced substantially continuously, iLe. as a cloud at least partlysuspended by the gaseous environment within the barrel, between 'thedischarge chamber and its mouth while a train of pulses was suppliedagainst the electrode so that the resulting sequence of dischargesimparted intermittent but repeated high-energy rate forces to theparticles and impelled them toward and against the workpiece surface. Inupright positions of the barrel, 1 provided frangible foil-typediaphragms as supports for the pulverulent material, the latter merelyresting upon the diaphragms. The needle electrode was constituted ofaluminum, zirconium, magnesium and copper (in this order of preference)since these materials appear to impart greater kinetic'energy to theparticles when used as discharge electrodes. correspondingly, foils ofaluminum, zirconium, magnesium, copper and nickel, have been found to beeffective as counter'electrodes.

- It was also pointed out there that means can be provided to heat theparticles to temperatures less than their fusion point but relativelyelevated by comparison'w'ith ambient temperature and, if possible, abovethe softening temperature of the described provided for the passage of aheating current through the mass of particles in advance of thedischarge, the useof externally operable electricheating means, themixing with the particles of a reducing agent capable of promoting anoxidation-reduction reaction with'theparticles during impulsivepropagation of the mass in thedi-rection of the substrate. It was foundthat the incorporation of 'a reduction-oxidation reaction system in theparticulate mass is highly effective since the reactants tend to remainin a quiescent state until the generation of aispark discharge; thequiescent state terminates very shortly after the discharge and aheating reaction is initiated slightly before or concurrently withacceleration of the particles and their dispersion so that they'areheated without significant interparticle fusion.

lnboth of the parent applications of the present case, .1 haveemphasized the fact that a surprisingly firm and durable bond resultsfrom the use of spark generators as the sourceof impulsive energy. Thesurprising results apparently derive from the stripping of oxide layersfrom the -surfaces-of the particles or the destruction of bond-resistantsurface. skins. Thus practically all metallic particles having anoxideor other bond-resistant skin limiting interparticle bonding as well asparticleto-substrate adhesion can be joined together by the high-energyrate process in which a spark-type detonation source not only propelsthe particles in the direction of the substrate but also appears toeliminate the oxide layers and to pierce the bond-resistant surfaceskins.

In the latter application Ser. No. 629,633 (now Pat. No.

3,461,268), 1 have provided a system for increasing the higha energyrate propulsion of the particulate material by preventcation providedthat the particulate material be pocketed between a pair of metallicfoils which-thus form a laminate as well as counterelectrodes forjuxtaposition with the needle electrode. The apparatus thus comprised abarrel portion and a shockwave generator portion, these portions beingseparable to receive the pocketed foil between them. Advantageously, theportions are provided at their junction with sealing means cooperatingwith the foil so that the latter simultaneously forms apressure-retaining and self-locking sealing joint. The pocketing foil orfoils consisted of oneor more materials which were intended to be foundsubsequently upon the coated surface. It is particularlydesirable to usefor the foil material a substance which is readily bendable both to theparticles and to the substrate inasmu'ch'as a substantial portion of thefoil is found to be present at the interface between the particles andthe substrate. For exampleyl employ a nickel foil when tungsten carbideor like hard-facing material is to be bonded to steel or the like. Itappearsthat the nickel acts as a bonding layer between particles of thehard-facing material and hot substrate and derives from theifoiloriginally employed to retain the particles. It is also conceivable tosubstitute for loose masses of the particles in the pockets of the foillayer, to lightly sinter or adhesively' bond the particles in moldedcoherent masses along a continuous foil and to the latter. Theinterparticle bond should, of.course, be as little as possible so as toconserve shockwave energy.

It is the principal object ofthe present invention to carry forwardprinciples originally disclosed and inherent in the aforementionedcopending applications.

Another object of this inventionis to provide improved means forpropelling particulate material against a substrate was to effect afirmer bond between the particles and the substrate and increase thequantity of material bonding to the latter.

Another object of this invention is to provide an improved method ofpatterning a surface using principles in part disclosed in the earlierapplications and above.

Thus, from subsequent experimentation with systems of the type describedand claimed in the aforementioned copending application, I havediscovered that the preheating of the particles plays a highlysignificant role in .the degree of bonding to the surface and in theproportion of the material which adheres firmly to the substrate;additionally, it appears that electrically subdivided particles are morereadily adherent and penetrate more effectively into' the substratesurface as is described in greater detail below.

According to a more specific feature of this invention, the particulatemass is formed in situ within the barrel of the discharge chamber bythermal destruction of a fusible material, the thermal destruction beingeffected by electrical disin tegration or erosion of the fusible elementby hot gases, preferably in a plasma condition. In'accordance with thisaspect of the invention. I may provide a pair of particle-formingelectrodes at a location ahead of the discharge electrode and heat theseparticle-forming electrodes by electrical resistance or arc-formingtechniques to vaporize the metal of at least one of these electrodes andform particles which are totally gaseous in nature or, upon condensationor solidification at the temperature within the discharge chamber, arein a liquid or solid finely subdivided state. In effect, therefore, theparticle cloud produced in this manner is a condensate of a particlesize substantially smaller than the particles of similar materials madeby mechanical techniques. Still another feature of this aspect of theinvention resides in heating a mixture of wire by arc discharge orresistance heating and generating the impulsive particles in propagatingdischarge when the heated portion of the fusible wire is only slightlycoherent so that the energy of the discharge first disrupts the heatedbody and breaks it into the particles of liquid or semisolid materialand thereafter entrains or propels these particles against thesubstrate. In a system ofcorresponding effectiveness, a plasma gun isprovided to inject a particle cloud contained in a hot plasma into thedischarge chamber just ahead of the electrode. In the system ofapplication Ser. No. 574,056, I have forecasted this modification bythere providing the particles in a free-falling mass from a hopper viaconventional dispensing means; in accordance with the present invention,however, I find it preferable to introduce the particles by entrainingthem in a gas, preferably a plasma as indicated earlier although asimple air stream may be satisfactory as described hereinafter. Such asystem represents a vast improvement over prior flame-plating"processes.

According to another aspect of this invention, a magazine is providedfor successively locating masses of the particles ahead of the dischargeelectrode, this ,magazine being constituted by a horizontal turntable ordisc composed of foil which, after the disc has been destroyed, isremoved from its support and replaced by another disc carrying pocketedmasses of particles or merely piles of the particles ofa flat surface.

Another feature of the present invention involves thesurprisingdiscovery that a minimum repetition rate of the order of 0.5to l cycle per second of the spark discharges in the impulse generatoris necessary to provide a satisfactory degree of deposition upon ametallic substrate. Thus, while one would ordinarily believe that thequantity of particulate material deposited upon the substrate is afunction only of the surface characteristics of the substrate, thetemperature of the detonation generator (see application Ser No.629,633), the character of the particles and the energy of thedischarge, I have found in subsequent experimentation that a surprisingincrease in the quantity of particles developed per unit powerconsumption is obtained when the spacing between pulses of the generatordecreases from a frequency of 0.5 cycles/second to a level which may beof the order of kilocycle/second. As a "acal matter. however, impulsesmay be triggered at a rate CO 500 cycles/second, depending upon the rateat which particles can be fed to the gun. Thus, optimum deposition isobtained when a pulse frequency (with corresponding interpulse spacingor delay) of0.5 to 500 cycles/second is used. Of course, the pulsefrequency must be less than that at which continuous discharge isgenerated across the spark electrodes.

Still another feature of this invention resides in the use of theprinciples described above and theaforementioned copending applicationsand their predecessors for the patterning of workpiece surfaces. Theterm patterning as used herein, is intended to refer to the formation ofdesigns, textures, color distributions and imprinting on metallic orother workpieces. For example, I have found that detonation typespark-discharge waves may be used to propel synthetic resin particles ina slightly preheated state ag inst paper or synthetic-resin substrateswhich have been electrostatically charged in accordance with apredetermined pattern to thereby fix the particles to the surface evenwithout the aid of heat. Electrostatic charges may, in part, repel theparticles of opposite charge directed against the surface from the pairof electrodes at which the particles are formed by electroerosion.Alternatively, a stencil, mask or the like may be disposed between theparticle-receiving surface of the workpiece and the impulse generator toform patterns upon the workpiece in accordance with the openings in themask or stencil. Still other patterning possibilities may make use ofthe fact that a magazine like supply of particles in doses to theimpulse generator may make use of particles in the respective doses ofdifferent color so that, especially when a pencil is coupled with theturntable, for example, patterns having differently colored areas may beformed on the workpiece. According to yet another specific feature ofthis aspect of the invention, the colored particles are formed in situin a pigment-producing reaction from, for example, a metallic rod.Particles of two or more metals oxidized to a predetermined colorationlevel, can be formed by effecting an arc discharge between the electroderods ahead of the impulse generator. When a plasma-em trained particlecloud is supplied to the impulse generator as described broadly above,the plasma itself may form the counterelectrode for the impulsegenerator, the ionizing source for triggering the discharge, etc.

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accom' panying drawing inwhich:

FIG. 1 is an axial cross-sectional view of an apparatus embodying theprinciples of the present invention;

FIG. 2 is an axial cross-sectional view of a modified system fordepositing particles upon a substrate;

FIG. 3 is still another cross-sectional view through a coatingapparatus;

FIG. 3A-3D are graphs illustrating an aspect of the invention; I

FIG. 4 is an axial cross-sectional view through a magazine typedeposition device;

FIG. 4A is a section along line IVA-IVA of FIG. 4; and

FIG. 5 is a cross-sectional view in diagrammatic form of a system usinga plasma torch for supplying the particulate material to the dischargegun. V

In the system of FIG. 1, the discharge chamber is formed as a barrelwhose mouth 101 is trained to the surface 102 of a substrate 103 whichcan be either conductive or nonconductive. A gap 104 is provided aroundthe zone of the surface 102 surrounded by the barrel 100 topreventpressure increases therewithin from reducing the kinetic energy of theparticles projected against the surface 103 at the other end of thebarrel 100, an insulating block 113 receives a needletype electrode 112which can be threaded into the barrel 100 axially to a variable distancer from the region at which a hopper 114 feeds the pulverulent materialinto the barrel transversely. Thus, a cloud of particles 105 is formedbetween the detonation-wave generator formed by electrode 112. Thehopper 114 is provided with a feeding or metering mechanism 115 whosemotor 116 is driven intermittently by a timer 117 which -tor 607. Aswitch 609 is triggerable as also controls a switch 109 in the supplycircuit for the gun which may be adapted to deposit a hard-facingmaterial upon the workpiece 103. The supply circuit 106 comprises adirectcurrent source (shown as battery 103) across which is bridged acapacitor ,107 in series with a charging resistor-110. The

distance 1 is adjusted in this embodimentuntil closure of switch 109will result in a discharge behind the mass of particles 105 whosepresence modifies the breakdown voltage which must be applied betweenthe needle 112 and the barrel 100 across which the pulsing source 106 isconnected. When larger quantities of conductive powder .105 are suppliedin the region of electrode 112, or the particle, cloud isdelivered froma plasma generator-(cf. FIG. the-breakdown voltage-is reduced and rapidpulses can be supplied so that a train of discharges, at a repetitionfrequency determined by the timer 117 and synchronized with the particlefeed means, can drive the particlecloud against the surface-102. Ingeneral, the discharge takes place rearwardly of the'particlemass 105and among these particles to partially ionize them, strip their oxidefilms and effect direct transfer of kinetic energy to the particles. Itwill also he understood that the'timer means need not be used inasmuchas the closure of switch 109 will apply a given potential between theneedle 112 and the barrel 100 and that the wiring of the discharge canbe initiated either by advancing the needle 112or by introducing asufficiently large mass of the conductive particles 105 oiwsupplyingthese particlesina plasma cloud. I

In FIG. 2, I show a system wherein' the particulate material is preparedfrom at least one continuous fusible element with the aid of arcdischarge or plasma and then is subjected to propulsion by the shockwave of a spark impulse generator. This system is particularlysatisfactory because it permits. high repetition rates to be attained.The barrel 600 of FIG. 2 opens in. the direction of theparticle-receivingsurface 601 of the workpiece 603 and embodies a pairof "arc-discharge electrodes 615 which are connected in series with achoke 615a and an AC source 6151) tosustain'a continuous arc dischargebetween these electrodes. The electrodes may consist of vaporizable wireand may be electrically decomposed sothat vapors of the fusible materialof the electrode wire, upon condensation, form a particle mass 605 Thezparticles are driven against the surface 602 by a spark dischargefrom a needle electrode 612, which may be advanced 'by a motor 612aenergized by a pulse source 606 whose battery 608 is connected incircuit with a charging resistor 610 anda discharging capacidescribedearlier to operate the impulse generator.

EXAMPLEJI Using the apparatus so far described in connection with FIG.2, one of the arc electrodes 615 was composed of a sintered material (85percent by weight tungsten carbide, 5 percent by weight iron and 10percent by weight nickel) while the other are electrode 615 was purenickel. Each electrode has a. diameter of 5 mm. and a length of ISO mm.A. DC are discharge at volts and 40 amperes was passed across theseelectrodes to effect fusion of them. Using. the system 606, 612

of FIG. 2, a spark discharge was triggered at a location 40 mm. behindthe gap between the electrode61 5,- the spark discharge having 6000joules energy and. a pulse width of l 10 microseconds. The workpiece 603was a sheet of 555C carbon EXAMPLE u microseconds, three such sparksbeing produced with each spark having an energy of about 2000: joules.Instead of con tinuous spark discharge between the electrodes 615, anintermittent discharge was provided in synchronization with the sparks.The resulting layer upon the workpiece 603 had a thickness of 100microns and the hardness specified injExample I. In both cases, the wearresistance of the surface .was increased from 8- to l0-times.

It will also be understood that the same principle applies if a fusiblewire is provided aside from the arc electrodes 615.

Thus, the wire 615a may be continuouslyfed from a supply reel 615dbetween the erosion electrodes 615 which are of a refractory metal anddo not materiallyferode during thepresent invention, this systemcomprising a barrel 700 directed toward the workpiece 703 'andi composedof an elec- 'trically and thermally insulating material in which anannular electrode 724 is embedded. Electrode 724 cooperates with anadjustable electrode 7l2'as previouslydescribed to produce a Followingthe method describedin Example I, intermittent spark discharges are usedwith f'apulse width of 2.l

discharge, behind a powder cloud705 formed by air injection of powderthrough the nozzle7 l5, A rnixing chamber'7l5a' is represented indiagrammatic formwhile'the control trigger or timer 717 is shown at 716to regulate both the switch 709 and the proportioning of powder and air.The discharge source 706 here includes a battery 708, a resistor 710 anda discharge capacitor 707. a

' EXAMPLEIII Using the apparatus of FIG. 3, tests were made with variousparticulate materials to ascertain'the relationship of depositionquantity firmly bonded to the S55C carbon steel workpiece. FIGS. 3A3D,in which the ordinate shows the quantity of material deposited (in'milligrams) and the abscissa, plotted in logarithmic scale, representsthe repetition rate in cycles per second. FIG. 3A shows a deposition oftungsten carbide powder after ten discharges, each with 0.l g of powderand 3000 joules spark energy. The graph shows a sharp rise in thedeposition quantity in the range of 0.5 to I cycle/second.

FIG. 3B similarly makes use of aluminum oxide powder with energy of 5000joules-per-dischar'ge, the same.marke d1in-' crease in depositionquantity being revealed. In FIG. 3C t he joules discharge energy areshown in FIG. 3D. While, with tungsten powder, the rate of increase ofthe deposition quantity with increasing repetition rate is less thanthat'obtained .with the other powders described, a substantial increasenevertheless is seen to take place at the critical region of 0.5--lcycle/second.

In FIGS. 4 and 4A, I show a system for, the repeated powder depositionupon a surface 802 of a workpiece 803. In this case, the barrel 800 ofthe gun is provided with an opening 800a through which a rotary disc820, composed of metal foil and carrying individual doses 805-ofparticles of different stencil 820cis rotated synchronously with themagazine 820 so that each color forms its own pattern onthe surface 802.

In the embodiment shown in FlG.5,;jtheI- barrel 900 faces the workpiece903 and is composed of a thermally insulating and electricallynonconductive material. The powder is here introduced in a plasma cloud905 ahead of the discharge electrode 912 which is axially shiftable inthe barrel 900 and may receive electrical impulses from a capacitor 907charged in the manner previously described, the spark discharge beingtriggered by a switch 909 operated by a timer (FIG. 1). The capacitor907 may be charged by a DC source in the usual manner (FIGS. 1-4). inthis case, the powder-containing plasma cloud 905 is injected into thebarrel 900 from a plasma gun 9l5e. Such guns are commonly employed asplasma torches (HO. 2) and have an annular electrode 915f coaxial with acentral electrode 915g which defined a chamber 9l5h with the outerelectrode. The nozzle 915i is cooled by water circulating through thepassage 915j. A high-temperature arc is sustained in the chamber 915hand an inert gas may be introduced with or without powder at 915k tothis chamber for conversion into the plasma. The term plasma" is usedherein in the sense considered conventional in the plasma-torch arc andrefers to a torch in which the emerging gases are ofa temperature suchthat a substantial portion of the emergent fluid is thermally orelectrically ionized. Powder may also be introduced into the gas closeto the passage 915i via a duct 915m. it will be understood that theplasma injection means can be coaxial with the barrel 900 in a variantof the modification described. As discussed in connection with FIG. 1,the plasma may, if pulsed, serve as the sole means for controlling thespark discharge and for triggering the device (switch 909 beingpermanently closed or eliminated).

The invention described and illustrated is believed to admit of manymodifications within the ability of persons skilled in the'art, all suchmodifications being considered within the spirit and scope of theappended claims.-

1. An apparatus for depositing particulate material upon a receivingsurface of a substrate, comprising housing means having a shockwavegenerator trained on said surface, means between said shockwavegenerator and said surface for introducing a cloud of particles into thepath of a shock wave propagated from said generator toward said surface,means for triggering a spark discharge in said generator to produce saidshockwave, and means for controlling the distribution of said particlesonto said surface to pattern the latter.

2. An apparatus for depositing particulate material upon a receivingsurface of a substrate, comprising housing means having a shockwavegenerator trained on said surface, means between said shockwavegenerator and said surface for introducing a cloud of particles into thepath of a shock wave propagated from said generator toward said surface,means for triggering a spark discharge in said generator to produce saidshock wave, the means for introducing said cloud of particles into thepath of said shockwave including a fusible body, and means for thermallyeroding said fusible body.

3. An apparatus as defined in claim 2 wherein the last-mentioned meansincludes electrode means for eroding said body by are discharge.

4. An apparatus as defined in claim 2 wherein the last-mentioned meansincludes a plasma gun trained at said body.

5. An apparatus for depositing particulate material upon a receivingsurface of a substrate, comprising housing means having a shock wavegenerator trained on said surface, means between said shock wavegenerator and said surface for introducing a cloud of particles into thepath of a shockwave propagated from said generator toward said surface,and means for triggering a spark discharge in said generator to producesaid shock wave,'the means for introducing said particle cloud into saidpath including a plasma gun forming a plasma stream entraining saidparticles.

6. An apparatus for depositing particulate materials upon a workpiecesurface, comprising electrode means forming a spark-discharge impulsegenerator trained on said workpiece, a disc of frangible materialinterposed between said generator and said workpiece and carrying atangularly spaced locations therealong res ective masses of particulatematerial, means for successive y with said generator, and means fortriggering said generator upon each alignment of a respective mass withsaid generator to deposit the particles of successive masses upon saidsurface in succession.

aligningsaid masses of particulate material

2. An apparatus for depositing particulate material upon a receivingsurface of a substrate, comprising housing means having a shockwavegenerator trained on said surface, means between said shockwavegenerator and said surface for introducing a cloud of particles into thepath of a shock wave propagated from said generator toward said surface,means for triggering a spark discharge in said generator to produce saidshock wave, the means for introducing said cloud of particles into thepath of said shockwave including a fusible body, and means for thermallyeroding said fusible body.
 3. An apparatus as defined in claim 2 whereinthe last-mentioned means includes electrode means for eroding said bodyby arc discharge.
 4. An apparatus as defined in claim 2 wherein thelast-mentioned means includes a plasma gun trained at said body.
 5. Anapparatus for depositing particulate material upon a receiving surfaceof a substrate, comprising housing means having a shock wave generatortrained on said surface, means between said shock wave generator andsaid surface for introducing a cloud of particles into the path of ashock wave propagated from said generator toward said surface, and meansfor triggering a spark discharge in said generator to produce said shockwave, the means for introducing said particle cloud into said pathincluding a plasma gun forming a plasma stream entraining saidparticles.
 6. An apparatus for depositing particulate materials upon aworkpiece surface, comprising electrode means forming a spark-dischargeimpulse generator trained on said workpiece, a disc of frangiblematerial interposed between said generator and said workpiece andcarrying at angularly spaced locations therealong respective masses ofparticulate material, means for successively aligning said masses ofparticulate material with said generator, and means for triggering saidgenerator upon each alignment of a respective mass with said generatorto deposit the particles of successive masses upon said surface insuccession.